Magnetic resonance imaging (MRI) is a common modality for imaging joints and other parts of the body due to its excellent definition of ligaments, cartilage, bone, muscle, fat and superior soft tissue contrast. Many MR techniques have been able to provide information about late stages of degeneration in which structural defects are present.
When a substance, such as human tissue, is subjected to a uniform magnetic field, the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field that is in the x-y plane, and which is near the Larmor frequency, the net aligned moment, or “longitudinal magnetization,” may be rotated, or “tipped,” into the x-y plane to produce a net transverse magnetic moment. A signal is emitted by the excited spins after the excitation signal is terminated, and this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients are employed. Often, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received nuclear magnetic resonance (NMR) signals are digitized and processed to reconstruct the image using a reconstruction technique.